Acoustic Model 115 - Reverb Not Working

[Gahoo]

Hey all,

Got an Acoustic Model 115 combo amp (solid state) on the bench. I was told the reverb wasn't working so I took the pan out of the chassis. I made sure the input and output cables to the pan are making good contact with the RCA jacks and tapped the springs to be sure the recovery and "Reverb" knob were working. So now I've got it reduced to an issue with the driver circuit.

Here is a schematic of that portion.



I tested both transistors for continuity/voltage drop and everything looked good. Checked all three 100uF electros and all are within spec.

On the right side of R61 I get 70.5 VDC (Schematic shows 76)
On the left side I get 45.4 VDC (Schematic shows 38)

For the transistors my voltages are as follows:
Q14 Emitter: 38 VDC
Q14 Base: 37.3 VDC
Q14 Collector: 38 VDC

Q13 Emitter: 37.2 VDC
Q13 Base: 26.75 VDC
Q13 Collector: 45.5 VDC

I'm lost at where to check from here. Are the discrepancies between my readings and the schematic enough to turn off the transistors? Something else I'm missing?

Thanks in advance!

[Jazz P Bass]

From here it appears that Q14 is toast.

[Gahoo]

Would've been my guess as well, thinking it was a short between the emitter and collector. No short between the two points though. Tried a different PNP and am still getting the same readings.

Tracing through the driver I get signal on both sides of C29 but after R54 (same node as the base of Q13) I get noise that looks like this.



R54, R55, and R56 all test within spec.

[Jazz P Bass]

Ok.

Did you sub Q13?

That Emitter voltage looks awfully high.

[52 Bill]

All of your voltages are off. Have you checked the ground connection to the circuit section?

[Gahoo]

All of your voltages are off. Have you checked the ground connection to the circuit section?

Grounds all check out, have continuity with the chassis.

Ok.

Did you sub Q13?

That Emitter voltage looks awfully high.

Swapped both Q14 and Q13 out for new transistors. Still no signal at output of drive section.

New voltages are as follows

Q14 Base 36.7V
Q14 Collector 37.3V
Q14 Emitter 37.4V

Q13 Base 29.2V
Q13 Collector 44.8V
Q13 Emitter 36.7V

Thanks for the help so far!

[Enzo]

Is R59 open? And does connecting the reverb pan affect these voltages? The drive end of the reverb pan has a very low DC resistance, and even in parallel with that 3.9k we are still talking a low resistance. Without that low resistance (like when the pan is disconnected) the voltage drop across R59 would be large.

[Gahoo]

Didn't even think about not having the pan connected...

Plugged it in and all is well. Thanks everyone!